Electronic equipment is equipment formed by disposing electronic parts on an external circuit such as a printed circuit board so as to perform their functions. From long in the past, electronic parts have been connected to external circuits such as printed circuit boards using solder because connection can be carried out at a low temperature, solder is inexpensive, and it produces high joint reliability. Electronic parts used in electronic equipment are provided with terminals such as part leads for connecting the parts to printed circuit boards.
Solder is also used inside electronic parts in locations where part elements, which perform their functions of electronic parts, are connected to part leads and the like. In this specification, soldering inside electronic parts in this manner will be referred to as “internal soldering” of electronic parts, and solder which is used for this purpose will be referred to as “internal joint” solder for electronic parts. Solder joints which are formed in this manner will be referred to as “internal solder joints”.
Solder alloys used for soldering of electronic parts are alloys which have a composition close to 60% Sn and a low melting temperature among alloys of Sn and Pb. In particular, a solder alloy having a composition of Sn63-Pb37 has a solidus temperature and a liquidus temperature which are both 183° C. By using this solder, there is little occurrence of cracks at the time of cooling of the solder, and because it has the lowest melting point among all solder alloys of Sn and Pb, there is little damage to electronic parts due to heat. It is so widely used that the term “solder” generally refers to a Sn63-Pb37 solder alloy. However, during assembly of electronic equipment, heating also takes place in order to solder electronic parts to external circuits. If this Sn63-Pb37 solder alloy is used for internal soldering of electronic parts, due to the heating at this time, solder melts inside the electronic parts and causes shorts, part elements and part leads which were joined by solder separate from each other, and the electronic parts can no longer function. Therefore, solders for internal soldering of electronic parts are ones having a higher melting temperature than a Sn63-Pb37 solder alloy. These solder alloys are referred to as high-temperature solders because they have a higher melting temperature than solder alloys such as Sn63-Pb37 used for soldering printed circuit boards.
The compositions of conventional high-temperature solders used for internal soldering of electronic parts mainly have Pb as a main component and include Pb-10Sn (solidus temperature of 268° C. and liquidus temperature of 302° C.), Pb-5Sn (solidus temperature of 307° C. and liquidus temperature of 313° C.), Pb-2Ag-8Sn (solidus temperature of 275° C. and liquidus temperature of 346° C.), Pb-5Ag (solidus temperature of 304° C. and liquidus temperature of 365° C.), and the like. Because the solidus temperature of these high-temperature solders is at least 260° C., soldered portions inside electronic parts which are soldered with the above-described high-temperature solders do not melt at the time of soldering of printed circuit boards even when the soldering temperature is 230° C., which is somewhat higher than that usually occurring when using a Sn63-Pb37 eutectic solder for soldering printed circuit boards
In recent years, because the toxicity of Pb is becoming a problem, so-called lead-free solder is being increasingly widely used. At present, widely used lead-free solders include Sn-3Ag-0.5Cu (solidus temperature of 217° C. and liquidus temperature of 220° C.), Sn-8Zn-3Bi (solidus temperature of 190° C. and liquidus temperature of 197° C.), Sn-2.5Ag-0.5Cu-1Bi (solidus temperature of 214° C. and liquidus temperature of 221° C.), and the like. These lead-free solders have a melting temperature which is close to 40° C. higher than that of a conventional Sn63-Pb37 solder alloy.
However, even if one considers using a high-temperature lead-free solder for initial soldering, due to regulations on Pb, there has been no high-temperature solder having Sn as a main component and having a solidus temperature of at least 260° C. For example, in the case of a Sn—Ag based solder having a solidus temperature (eutectic point) of 221° C., the liquidus temperature increases if Ag is increased, but the solidus temperature does not increase. If the Sb content of a Sn—Sb based solder having a solidus temperature of 227° C. is increased as much as possible, the liquidus temperature greatly increases. Even if other elements are added to these alloys, the solidus temperature increases, and this tendency cannot be changed. Therefore, it has been thought impossible to use a lead-free solder as a high-temperature solder for internal soldering of electronic parts.
In Patent Document 1, as an invention of a solder alloy which is a replacement for a Pb—Sn high-temperature solder, the present applicant disclosed a solder paste which is characterized by comprising a high-temperature solder powder and a flux containing a thermosetting adhesive. The solder paste has Bi as a main component, it has a solidus temperature of at least 260° C. and a liquidus temperature of at most 360° C., and it is selected from Bi powder or Bi—Ag based powder, Bi—Cu based powder, Bi—Sb based powder, and Bi—Zn based powder.
Patent Document 2 discloses a solder composition comprising Bi, Ag as a second element, and a third element selected from Sn, Cu, In, Sb, and Zn.